If we are going to treat other species so very differently from our own — killing, eating and experimenting on pigs and sheep, for example, but never human beings — then it seems we need to come up with some morally relevant difference between us and them that justifies this difference in treatment. Otherwise it appears we are guilty of bigotry (in just the same way that someone who discriminates on the basis of race or sex is guilty of bigotry). But what is this morally relevant difference? Julia Tanner's article examines, and rejects, some of the most popular answers to this question.